1. Field of the Invention
Aspects of the present invention relate in general to an image forming apparatus, and more specifically, to a photosensitive drum with an improved shutter structure and an image forming apparatus having the same.
2. Description of the Related Art
In general, an image forming apparatus such as a laser printer, a copy machine, a scanner, a fax machine, etc., has a photosensitive drum unit for forming a latent image by laser beams on the surface thereof. The photosensitive drum, by its nature, is damaged when exposed to a light source for an extended period of time. Additionally, the photosensitive drum is protected by a shutter because it may easily be scratched when it is separated from an image forming apparatus.
FIG. 1 is a perspective view of a typical laser printer as one example of an image forming apparatus. Referring to FIG. 1, the laser printer includes a photosensitive drum unit 2, which is detachably installed through a front door 5 of a printer main body 1. The photosensitive drum unit 2 includes a photosensitive drum 2a where a latent image is formed by laser beam exposure, and a shutter 2b for covering the photosensitive drum 2a. When the photosensitive drum unit 2 is installed into the printer main body 1, the shutter 2b should be opened so that the transfer roller 4 and the photosensitive drum 2a can come in contact with each other, and also so that laser beams can be scanned onto the photosensitive drum 2a. However, when the photosensitive drum unit 2 is separated from the main body 1, the shutter 2b should be closed to protect the photosensitive drum 2a from external shock. Sometimes, a user opens a side door 3 of the printer to remove jammed paper from a paper cassette 6 or replace a component. In this case, it is important that the shutter 2b remains closed to protect the photosensitive drum 2a against light coming from the outside. Thus, in a related art image forming apparatus, the shutter 2b is built to be opened and closed interlockingly with the opening and closing of the side door 3 of the printer so that when the side door 3 is opened, the shutter 2b closes, and vice versa.
In effect, a number of techniques for opening and closing the shutter of the photosensitive drum unit interlockingly with the side door of the main body of the image forming apparatus have been suggested. For instance, according to a laser printer disclosed in Korean Patent No. 395522 and as depicted in FIG. 2A, when a protrusion 3a on the side door 3 pushes a sliding member 11 to the right with the side door 3 closed, a pivotable bracket 2c of the photosensitive drum 2a is pushed in the direction indicated by the arrow “A” by a pivotable member 13 connected to a hinge, and the shutter 2b connected to the pivotable bracket 2c through a wire 2d is opened downwardly. In this state, the photosensitive drum 2a makes contact with a transfer roller 4, and the print operation is performed normally. On the other hand, when the side door 3 is opened, for instance, because of a paper jam, as shown in FIG. 2B, the transfer roller 4 and the photosensitive drum 2a are separated from each other, and the sliding member 11 is pressed by a spring 12, causing the pivotable member 13 to move in the direction indicated by the arrow “B”. The pivotable bracket 2c of the photosensitive drum unit 2 is pulled by a tension spring 2e, and the shutter 2b connected to the pivotable bracket 2c through the wire 2d is closed upwardly. In this sliding mechanism, a relatively large space is required for opening/closing the shutter because the shutter of the photosensitive drum unit is opened and closed according to the motion of the pivotable member 13 and the wire 2d. A large space is also needed for the reciprocating motion of the sliding member 11.
FIG. 3 is a perspective view of another example of a related art photosensitive drum unit. In this case, the opening/closing of a shutter 220 according to the opening/closing of a side door 3 are done based on a totally different mechanism, i.e., a gear system. Unlike the photosensitive drum unit in FIG. 2, a photosensitive drum unit 200 in FIG. 3 is built in a manner that the shutter 220 is mounted on a long circular-shaped rotation axis 221, which is inserted into holes formed in a front and a rear supporter 203, 204, respectively, of a housing 201. A circular-shaped gear 211 is attached to one end of the rotation axis 221, and the gear 211 is also in mesh with a sector-shaped gear 210. The number of teeth of this sector-shaped gear 210 corresponds to the number of teeth required for the circular-shaped gear 211 to turn 360 degrees, or at least turned by a predetermined angle. Moreover, an arm 212 protrudes from one side of the sector-shaped gear 210, and a twist spring 207 is installed at a hinge shaft 208. One end of the twist spring 207 is connected to the sector-shaped gear 210, and the other end of the twist spring 207 is connected to the rear supporter 204 of the housing 201. According to the operational principle of this particular photosensitive drum unit 200, when the side door 3 is closed and the arm 212 of the sector-shaped gear 210 is pushed by a protrusion 3a protruding from the side door 3, the sector-shaped gear 210 rotates in the clockwise direction and causes the circular-shaped gear 211 to correspondingly rotate in the counterclockwise direction at the same time. As a result, the shutter 220 is opened upwardly. On the other hand, when the side door 3 is opened, the sector-shaped gear 210 returns to its original position by the twist spring 207, and the circular-shaped gear 211 also returns to its original position while rotating in the clockwise direction. The shutter 220 then closes downward to protect the drum 202.
In this gear system of FIG. 3, if the hinge shaft 208 of the side door 3 is mounted at a lower point of the side door 3, the radius of rotation of the side door 3 is increased, and therefore the length of a protrusion mounted at the side door also needs to be increased. A large installation space is therefore required for the shutter as long as the opening and closing of the shutter corresponds to the opening and closing of the side door. This fact makes it more difficult to manufacture a small-sized image forming apparatus.